It is known in the art to measure the free volume of a confined space by applying to the gas within the space repeated compressions of predetermined waveform by means of a diaphragm, for example, of a loudspeaker, and generating electrical signals representative of the resultant variation of pressure by means, for example of a microphone, and calculating the free volume of the confined space measuring the electrical signals so generated. See, for example, U.S. Pat. Nos. 4,561,298, for an invention of Pond, and 3,237,451, for an invention of Haeff; these patents are hereby incorporated herein by reference. In accordance with this approach, a reference container of known volume may be disposed adjacent to the unknown volume, and a common loudspeaker at the boundary between the two volumes may be used to apply repeated compressions simultaneously to both volumes. The resultant pressure variations can then be monitored by microphones placed in each volume, and the electrical outputs from the two microphones may be compared to obtain a signal whose amplitude is representative of the unknown volume. Pond (col. 2, lines 26-29) teaches that the frequency of the repeated compressions should be sufficiently low to avoid problems associated with Helmholtz resonance.
The accuracy of volume measurement systems of the acoustic-pressure type disclosed in these two patents is dependent upon the degree to which differences between the two microphones can be eliminated or compensated and also upon the recalibration of the systems to take into account drift caused, for example, by changes in temperature and humidity or other pertinent characteristics of the system and of the air or other gas used in the measurement. The accuracy may also be adversely affected by gas leaks in the reference or unknown volumes.